1. Field of the Invention
The present invention relates to a highly moisture-containing soft contact lens with a moisture content as high as 50-70% and a method for producing it. More precisely, it relates to a method for producing moisture-containing contact lenses with high moisture content by polymerizing a monomer solution in the presence of a diluent which can be substituted with water, physiological saline or the like and substituting the diluent contained in the resulting polymer with water, physiological saline or the like to afford a moisture-containing contact lense with high moisture content, which method can afford moisture-containing contact lenses with high moisture content exhibiting, in particular, excellent transparency and high mechanical strength.
2. Related Art
Soft contact lenses are recognized to have good wear feeling because of the flexibility of the material from which they have been made. In particular, it is said that soft contact lenses with a higher moisture content can exhibit better wear feeling. However, moisture-containing soft contact lenses with high moisture content generally show poor mechanical strength and hence their handling is disadvantageously difficult.
Soft contact lenses are generally produced by the cutting and polishing method, i.e., cutting and polishing a piece of copolymer into a lens shape and swelling the lens shape copolymer with water. However, it is not easy to improve the production yield of lenses meeting the required specification in the cutting and polishing method because the copolymer is likely to absorb atmospheric moisture during the processing. Further, in addition to the above problem, the cutting and polishing method comprises a number of steps requiring operation preciseness such as the steps of cutting and polishing the base curve, the front curve and the edge of lenses and therefore it is not suitable for the production of contact lenses in a large scale.
On the other hand, as a method for producing contact lenses not requiring cutting and polishing and hence suitable for large scale production, the cast polymerization method utilizing a mold for producing contact lenses has been known. However, the polymerization in a mold having a small volume used for this method is affected and inhibited at the interface of the mold and monomer material by oxygen molecules absorbed on the mold, and thus mechanical strength and/or transparency of the lenses may be degraded. In particular, white turbidness may be caused. Moreover, when moisture-containing soft contact lenses of high moisture content are produced by this method, it is difficult to precisely control lens parameters due to large elongation of the lens material during the lens swelling process.
Under the circumstance described above, a method which utilizes the cast polymerization and, besides, can produce moisture-containing soft contact lenses of high moisture content with stable lens parameters has also been used recently. In this method, lens monomers are polymerized in the presence of a diluent not inhibiting the polymerization of the monomers and the diluent trapped in the resulting lenses is substituted with water, physiological saline or the like to afford contact lenses. In this method, swelling rate of the lenses can be suppressed and it becomes easy to precisely control the lens parameters.
For example, Japanese Patent Unexamined Publication No. Hei 6-289331 (referred to as "Reference 1" hereinafter) discloses a method for producing contact lenses by cast polymerization wherein hydrophilic monomers are polymerized in the presence of a water-soluble liquid selected from water, dioxane and dimethyl sulfoxide. It is recommended that the content of the water-soluble liquid in the polymerization mixture is 30-80 parts by weight per 100 parts by weight of copolymerizable monomers.
Japanese Patent Publication No. Hei 4-49093 (referred to as "Reference 2" hereinafter) discloses a method for producing contact lenses by static casting wherein acrylic or methacrylic monomers are polymerized in the presence of a mixture comprising a boric acid ester which can be substituted with water. As the boric acid ester, boric acid ester of glycerol, sorbitol, propylene glycol, mixtures thereof and the like are exemplified.
According to Japanese Patent Unexamined Publication No. Hei 4-110311 (referred to as "Reference 3" hereinafter), hydrophilic (meth)acrylate ester monomers of a predominant monomer ratio, alkyl methacrylate whose alkyl group contains at least four carbon atoms and crosslinkable monomer are polymerized by cast polymerization in the presence of a diluent which can be substituted with water to afford contact lenses. The diluent in this method is boric acid ester of dihydric alcohols.
In the U.S. Pat. No. 3,699,089 (referred to as "Reference 4" hereinafter), soft contact lens are produced by spin casting and hydrophilic monomers are polymerized in the presence of water or a water-miscible solvent. The water-miscible solvent used in this method is ethylene glycol, glycerol, dioxane and the like.
As seen from the above-mentioned References, the solvent or diluent is appropriately selected depending on the kinds of monomers in order to obtain contact lenses optically transparent and having good mechanical strength.
According to the research of the present inventors, however, it was extremely difficult to obtain contact lenses optically transparent and having good mechanical strength by polymerizing hydrophilic monomers and alkyl methacrylates in the presence of a diluent (solvent) and substituting the diluent when any one of the diluents used in the methods described in the above References is employed. All of the methods of the above References could afford only lenses optically opaque or having poor mechanical strength.
Therefore, an object of the present invention is to provide a moisture-containing soft contact lens of high moisture content which is composed of a polymer of hydrophilic monomers and alkyl methacrylates and optically transparent and exhibits excellent mechanical strength and a method for producing it.